A biometric authentication technology has been developed authenticating a user of an apparatus or a system based on biometric image in which biometric information such as vein pattern of hands or fingers, fingerprints, or palm prints is represented. In the biometric authentication apparatus using such a biometric authentication technology, for example, the biometric information image-capturing device photographs a body part including the biometric information of the user who intends to use the biometric authentication apparatus to acquire a biometric image which represents the biometric information. Also, the biometric authentication apparatus matches the input biometric information of the user represented in the biometric image with the registered biometric information represented in a biometric image of a registered user who is registered in advance. When it is determined that the input biometric information coincides with the registered biometric information based on a matching result, the biometric authentication apparatus authenticates the user as the registered user who has been authorized. Also, the biometric authentication apparatus permits the authenticated user to use an apparatus equipped with the biometric authentication apparatus or another apparatus connected with the biometric authentication apparatus.
There is a biometric information image-capturing device which includes an image capturing unit which photographs the biometric information along with a light source which illuminates the body part including the biometric information in order to have sufficient contrast in the biometric information. Also, the illumination light from the light source is diffusively reflected from the body part including the biometric information and the diffusively reflected light reaches the image capturing unit, so that a luminance distribution according to the biometric information is reproduced on the biometric image. For example, when the biometric information used in the biometric authentication is the vein pattern of hands or fingers, an infrared light emitting diode is used as an illumination light source. The infrared light emitted from the infrared light emitting diode is incident into and repeatedly scatters in hands or fingers to finally become a diffusively reflected light which emits from a skin surface. Some of light rays are absorbed by, for example, the vein in the course of a diffused reflection of light. Also, some of the diffusively reflected light rays reach the image capturing unit. The diffusively reflected light described above diffuses toward various directions and thus, the luminance on the image capturing unit generally becomes lower.
In the meantime, some of the illumination light rays emitted from the illumination light source are specularly reflected on the surface of the body part including the biometric information. Since the specularly reflected light is directed only toward a direction reflected at a reflection angle which is equal to an incidence angle with respect to the normal line of the surface of the body part, the specularly reflected light is brighter than the diffusively reflected light. Therefore, when both the specularly reflected light and the diffusively reflected light are incident on the image capturing unit, an unevenness of luminance occurs between an area on which the specularly reflected light is impinged and another area on which the specularly reflected light is not impinged. Further, in some cases, the specularly reflected light becomes too bright to exceed a dynamic range of the image capturing unit in the area on which the specularly reflected light is impinged and thus, there is a concern that the luminance distribution of the diffusively reflected light representing the biometric information may not be reproduced. Therefore, a technology for removing the specularly reflected light which reaches the image capturing unit has been suggested. See, for example, Japanese Laid-open Patent Publication No. 2002-200050 and Japanese National Publication of International Patent Application No. 2002-514098.
Japanese Laid-open Patent Publication No. 2002-200050 discloses a skin color measurement apparatus in which each of an incident light and a reflected light from a light source is transmitted through a polarization plate, and the reflected light is detected to exclude a surface-reflected light reflected from a surface of skin. The skin color measurement apparatus includes two types of the polarization plates having different polarization directions and switches the polarization plate which transmits light between a case of measuring the surface-reflected light and another case of measuring the diffusively reflected light. Further, Japanese National Publication of International Patent Application No. 2002-514098 discloses a light emitting device including a plurality of light emitting elements that are able to selectively emit light and are positioned such that a regular reflection is excluded or minimized to avoid obtaining a blurred iris image.